Fire-sealing leads-through are commercially available in the form of prefabricated bodies with tubular shells having elastic, fire-resistant, foamed material accommodated therein, and compressed transversely of the longitudinal axis of the shell. These prefabricated bodies are intended for casting into walls and floors or prefabricated building components constituting such walls or floors. This means that it must be clear during the planning of a building which conduits are to be laid and how they are to be laid. When extending or rebuilding buildings as well as when increasing or altering the laying of conduits, the advantages provided by these prefabricated members cannot be utilized. It is then a question of attempting to seal the new holes which are made in walls and floors, using loose pieces of foamed material which are tamped in between the conduits in the holes. Alternatively, the seals can be molded in situ, but this is a complicated and time-consuming operation, if a satisfactory result is to be achieved.